The majority of condoms are made from natural rubber latex (NRL) by well known manufacturing processes. To ensure that condoms are suitable for use, their properties must meet the requirements of national, regional or international standards, which normally include a minimum burst pressure requirement.
While condom wall thickness is normally fairly low, being between 50 μm and 70 μm, it would be beneficial to reduce this even further to encourage the use of condoms. A perceived loss of sensitivity when using a condom is often used as an excuse for not using them, leading to an increased risk of pregnancy or sexually transmitted infections. Accordingly, it would be desirable to be able to manufacture thinner condoms. Not only is it desirable to make thinner condoms, these thin condoms must also meet the burst pressure requirements of the standards.
Although attempts have been made to make thinner condoms, it has not heretofore been possible to make thin condoms which meet the requirements for burst pressure specified in the standards.
The thinness of the condom is typically determined by the single wall thickness.
Single-wall thickness measurement on a condom is done via a weight measurement. A 20 mm ring section is cut from a parallel-sided part of the condom, preferably at the mid-body of the condom (such ‘ring’ sample pieces are typically used for tensile testing, and the thickness measurements are used in calculation of tensile strength). Knowing the circumference of the ring, its length of height and the density of the latex film, single-wall thickness can be calculated using the equation:
            Single-wall        ⁢                  ⁢    thickness    =      Weight          Density      ×      circumference      ×      height      ×      10      ,      000                      Where                    Single-wall thickness=thickness of one condom wall (μm)            Density=density of condom film (g/cm2)            Circumference=circumference of ring sample (cm)            Height=height of ring sample (cm)                        

The international condom standard (BS EN ISO 4074:2002 Natural latex rubber condoms: Requirements and test methods), along with many other standards, includes a requirement that condoms have a minimum burst pressure of 1.0 kPa when tested according to the method in the standard. In brief, the test requires that a condom is inflated at a fixed rate of flow of air, whilst both the pressure and volume of air in the condom are continuously monitored so that the pressure and volume readings when the condom fails by bursting are recorded. These measurements are known, respectively, as the burst pressure (measured in kilopascals, kPa) and burst volume (measured in liters, L, or decimeters cubed, dm3). This testing is carried out on a number of condoms from each batch, the number being determined by the batch size.
As the wall of the condom is made thinner, the pressure required to inflate and eventually burst the condom decreases. As a result, there is a lower limit to the condom wall thickness that can comply with the burst pressure requirements of the standard. Furthermore, inflation volume and inflation pressure are linked. Apart from the initial stages of inflation, the larger the inflation volume, the larger the inflation pressure for any condom type. Modulus is essentially a measure of stiffness, such that a lower modulus material is more pliable or elastic. Increasing the modulus (i.e. increasing the stiffness) of the condom material also increases the burst pressure when compared to a condom made from a material with a lower modulus, at the same inflation volume. That is, the higher the modulus of the condom material, the higher the pressure necessary to burst the condom, at a given inflation volume. However, in almost all cases, increasing the condom modulus has the additional effect of reducing the burst volume. Because burst pressure is related to burst volume, any reduction in burst volume will also lead to a reduced burst pressure. Thus, a condom made from a latex formulation that results in a lower burst volume will also have a lower burst pressure.
Previous attempts to produce a very thin condom complying with the burst pressure requirements have failed because use of materials with a higher modulus, in an attempt to maintain minimum burst pressures that comply with the required standards at low condom wall thickness, almost invariably causes reduced burst volume, which results in reduced burst pressure.
Two approaches have been tried in the past to achieve thinner condoms. Firstly, attempts have been made to make condoms from natural rubber latex (NRL) but using less NRL to give thinner condom walls. Secondly, attempts have been made to make condoms from synthetic materials having higher tensile properties than NRL.
In the first approach (using less NRL), there is a limit to how thin the condom walls can be before the condoms start failing to meet the requirements of the standards, and attempts to alter process parameters to alleviate this problem have been unsuccessful. It has been found that, in order to ensure that the manufacturing batch pass rate is as high as possible, the mean burst pressure of each batch typically needs to be at least two standard deviations above the minimum requirements given in the standards. This has been found to result in a minimum NRL condom thickness of between about 50 μm and about 55 μm (single wall thickness).
Using the second approach (using synthetic materials with superior tensile strength to NRL), it has been possible to make thin condoms. However, the synthetic materials used also tend to have higher low strain moduli and lower elongation-at-break than NRL and so the benefits of having a thinner condom, such as improved perceived sensitivity, are negated by these thinner synthetic condoms being perceived as being stiffer and less flexible, which is undesirable. As a result, these condoms made from synthetic materials are unsatisfactory.
Recent work with high-styrene styrene-butadiene rubber latex (SBR) and carboxylated SBR (X-SBR) gave reinforcement of tensile modulus but we have found that condoms made from NRL incorporating SBR and/or X-SBR suffer from the lower burst volumes described above. As a result, thin condoms made from NRL and SBR/X-SBR blends are not predicted to meet the burst pressure requirements of the standards.
The data shown in FIGS. 1 and 2 illustrate that although tensile strength is increased for X-SBR/NRL blends (FIG. 1) compared to NRL alone (FIG. 2), the addition of X-SBR has lowered the burst volume and, as a consequence, the burst pressure for condoms made from X-SBR/NRL blends (FIG. 1) is similar to that of the unmodified formulation (that is, without the addition of X-SBR (FIG. 2)) at similar condom thicknesses.
We have now found that it is possible to make NRL condoms that are significantly thinner than current NRL condoms and which have acceptable overall properties, such as, in particular, perceived stiffness, and meet the requirements of the standards, by blending a polyurethane latex with natural rubber latex.
The limitations described above, where reinforcement leads to higher modulus, lower burst volumes but no improvement in burst pressure have, surprisingly, been overcome by using polyurethane as a reinforcing blend with natural rubber in the manufacture of condoms.